Head-up Displays (HUDs) are typically used in aircraft to provide flight attitude information as referenced with respect to a heading of the aircraft. The term “heading,” or heading angle, is referred to herein as an angle associated with a longitudinal axis of the aircraft with respect to north. Primary control symbols presented on HUDs include a Flight Path Vector (FPV) and other earth referenced data such as airports, runways, and the like.
In comparison with HUDs, head-down displays generally have a visually smaller display screen, e.g., smaller in angle subtended at a pilot's eye. Head-down displays also display a wide variety of data that are generally not provided on HUDs. On head-down displays, symbology is generally not displayed overlaying actual outside objects (e.g., mountains, buildings, etc.), such as found with HUDs. Instead, synthetic analogs or enhanced sensor images of actual outside objects may be displayed in head-down displays. For example, flight attitude information may be projected over terrain symbology as if looking out of a cockpit window.
Wind and other forces typically influence an aircraft so that the aircraft heading may not coincide with an actual direction of travel, or track. The term “track,” or track angle, is referred to herein as an angle of a ground speed vector with respect to north. For example, track is the direction from north that the aircraft is moving. Most flying conditions encountered by pilots generate a track that is different from a heading of the aircraft. While using a head-down display that is oriented with reference to the heading of the aircraft, such as mimicking HUD, the pilot typically makes numerous flight adjustments to account for the track of the aircraft.
A head-down display is typically displayed in a heading-up/track-up, or hybrid centered mode. When displaying in hybrid centered mode, various approaches are needed to ensure smooth transitions as well as to provide sufficient awareness of heading and drift conditions. In particular, a displayed drift angle is displayed in a hybrid centered head-down display to aid the aircraft crew with flight attitude information. The term “displayed drift angle”, is referred to herein as the angle of separation between the track and a displayed centerline. The displayed drift angle is different from a conventional drift angle that is defined as the difference between the track and heading angles in both heading or track centered displays.
Accordingly, it is desirable to provide a hybrid centered head-down display including displayed drift angle limits, wherein displayed is earth referenced symbology, such as synthetic terrain, that is useful to a flight crew for flight management during different flight conditions. In addition, it is desirable to provide a method of calculating optimal displayed drift angles and displayed drift angle limits when displaying earth referenced symbology that will fit on a head-down display and that is useful for flight management during different flight conditions and reduces pilot workload. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.